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This article appeared in the
Awake! magazine of
May 22,
2003
When Insects Spread DISEASE
Insect-Borne Disease—A Growing Problem
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Black flies carry river blindness
Mosquitoes carry malaria, dengue, and yellow fever
Lice can convey typhus
Fleas host encephalitis and other diseases
Tsetse flies transmit sleeping sickness
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IT IS BEDTIME in a Latin-American home. A mother lovingly tucks her
young son in and bids him good night. But in the dark a sleek, black kissing
bug, less than an inch [less than 3 cm] long, slips out of a crevice in the
ceiling over the bed. It drops undetected onto the sleeping child’s face and
almost imperceptibly pierces the soft skin with its beak. As the bug gorges
itself on blood, it also discharges its parasite-laden waste. Without waking,
the boy scratches his face, rubbing the infected feces into the wound.
As a result of this one encounter, the child contracts Chagas’ disease. Within a
week or two, he gets a high fever and his body swells. If he survives, the
parasites may take up residence in his system, invading his heart, nerves, and
internal tissues. As many as 10 to 20 years may pass without symptoms. But then
he may develop lesions in his digestive tract, experience cerebral infection,
and ultimately die of heart failure.
This fictionalized account realistically depicts how Chagas’ disease can be
contracted. In Latin America, millions may be at risk of receiving this kiss of
death.
Man’s Multilegged Companions
“Most of the major fevers of man are produced by micro-organisms that are
conveyed by insects,” states the Encyclopædia Britannica. People commonly use
the term “insect” to include not only true insects—six-legged creatures such as
flies, fleas, mosquitoes, lice, and beetles—but also eight-legged creatures such
as mites and ticks. Scientists list all of these under the larger category of
arthropod—the largest division in the animal kingdom—which includes at least a
million known species.
The vast majority of insects are harmless to man, and some are very beneficial.
Without them, many of the plants and trees that people and animals depend on for
food would not be pollinated or bear fruit. Some insects help to recycle waste.
Many insects feed exclusively on plants, while certain ones eat other insects.
Of course, there are insects that annoy man and beast with their painful bite or
simply by their presence in vast numbers. Some also wreak havoc on crops. Worse,
however, are insects that spread sickness and death. Insect-borne diseases “were
responsible for more human disease and death in the 17th through the early 20th
centuries than all other causes combined,” states Duane Gubler of the U.S.
Centers for Disease Control and Prevention.
Presently, about 1 out of every 6 people is infected with a disease acquired
through insects. Besides causing human suffering, insect-borne disease imposes a
heavy financial burden, especially on developing countries—those that can least
afford it. Even a single outbreak can be costly. One such incident in western
India in 1994 is said to have drained billions of dollars from the local and
world economies. According to the World Health Organization (WHO), the world’s
poorest countries will be unable to advance economically until such health
problems are brought under control.
How Insects Make Us Sick
There are two main ways that insects serve as vectors—transmitters of disease.
The first is by mechanical transmission. Just as people can track dirt into a
home on unclean shoes, “houseflies may carry on their feet millions of
microorganisms that, in large enough doses, can cause disease,” says the
Encyclopædia Britannica. Flies can pick up contamination from feces, for
example, and pass it on when they land on our food or drink. In this way humans
contract such debilitating and deadly illnesses as typhoid, dysentery, and even
cholera. Flies also help to spread trachoma—the leading cause of blindness in
the world. Trachoma can blind by scarring the cornea—the clear part of the eye
in front of the iris. Worldwide, some 500,000,000 humans suffer from this
scourge.
Cockroaches, which thrive in filth, are also suspected of mechanically
transmitting disease. In addition, experts link a recent steep rise in asthma,
especially among children, to cockroach allergies. For instance, picture Ashley,
a 15-year-old girl who has spent many nights struggling to breathe because of
her asthma. As her doctor is about to listen to her lungs, a cockroach falls out
of Ashley’s shirt and runs across the examination table.
Diseases on the Inside
When insects harbor viruses, bacteria, or parasites inside their bodies, they
can spread disease a second way—by passing it on through a bite or other means.
Only a small percentage of insects transmit disease to humans in this way. For
instance, although there are thousands of species of mosquitoes, only those of
the genus Anopheles transmit malaria—the world’s second-deadliest communicable
disease (after tuberculosis).
Still, other mosquitoes transmit a host of different maladies. WHO reports: “Of
all disease-transmitting insects, the mosquito is the greatest menace, spreading
malaria, dengue and yellow fever, which together are responsible for several
million deaths and hundreds of millions of cases every year.” At least
40 percent of earth’s population are at risk for malaria, and about 40 percent
for dengue. In many places, a person can contract both.
Of course, mosquitoes are not the only insects that carry disease inside them.
Tsetse flies transmit the protozoa that cause sleeping sickness, afflicting
hundreds of thousands of people and forcing whole communities to abandon their
fertile fields. By transmitting the organism causing river blindness, blackflies
have robbed some 400,000 Africans of sight. Sand flies can carry the protozoa
that cause leishmaniasis, a group of disabling, disfiguring, and often fatal
diseases that presently afflict millions of people of all ages around the world.
The ubiquitous flea can host tapeworms, encephalitis, tularemia, and even
plague—generally associated with the Black Death, which in just six years wiped
out a third or more of the European population during the Middle Ages.
Lice, mites, and ticks can convey various forms of typhus, besides other
diseases. Ticks in temperate lands around the world can carry potentially
debilitating Lyme disease—the most common vector-borne illness in the United
States and Europe. A Swedish study revealed that migratory birds can transport
ticks thousands of miles, possibly introducing the diseases they carry to new
regions. “Ticks,” says the Britannica, “surpass all other arthropods (except
mosquitoes) in the number of diseases they transmit to humans.” In fact, a
single tick can harbor as many as three different disease-causing organisms and
can transmit all of them in just one bite!
A “Vacation” From Disease
It was only as recently as 1877 that insects were scientifically shown to
transmit disease. Since then, massive campaigns to control or eliminate
disease-carrying insects have been carried out. In 1939 the insecticide DDT was
added to the arsenal, and by the 1960’s insect-borne disease was no longer
regarded as a major threat to public health outside Africa. Emphasis shifted
away from controlling the vectors to treating emergency cases with drugs, and
interest in studying insects and their habitats waned. New medicines were also
being discovered, and it seemed that science could find a “magic bullet” to deal
with any illness. The world was enjoying a “vacation” from infectious disease.
But the vacation was to end.
Why the RESURGENCE?
ABOUT 40 years ago, classic insect-borne diseases like
malaria, yellow fever, and dengue were thought to have been nearly
eradicated from large areas of the earth. But then the unexpected
happened—insect-borne diseases began to reemerge.
Why? For one thing, some of the insects and the microbes they carry
have developed resistance to the insecticides and medicines used to
control them. This natural process of adaptation has been given a boost,
not only by overuse of insecticides but also by misuse of medicines. "In
too many poor households," states the book Mosquito, "people
obtain the medicine, use just enough to ease their symptoms, and then
hoard the remainder for the next wave of illness." With such an
incomplete cure, the stronger microbes may survive in a person's body to
produce a new generation of drug-resistant offspring.
A Change in the Climate
An important factor in the resurgence of insect-borne diseases is
change—in nature and in society. A case in point is global climate
change. Some scientists expect a warming global environment to expand
the range of disease-carrying insects into presently cooler climates.
There is some evidence that this may already be taking place. Dr. Paul
R. Epstein of the Center for Health and the Global Environment, Harvard
Medical School, notes: "Both insects and insect-borne diseases
(including malaria and dengue fever) are today being reported at higher
elevations in Africa, Asia, and Latin America." In Costa Rica, dengue
has spilled over the mountains, which until recently confined the
disease to the Pacific Coast, and it now blankets the entire country.
But warmer weather can do more. In some areas it transforms rivers
into puddles, while in others it triggers rains and floods that leave
behind stagnant pools. In both cases the standing water serves as a
perfect breeding ground for mosquitoes. Hotter weather also shortens the
mosquitoes' breeding cycle, speeding up their reproduction rate, and it
lengthens the season during which mosquitoes abound. In warmer weather,
mosquitoes are more active. Hotter temperatures even reach inside the
mosquito's gut and intensify the reproduction rate of disease-causing
microbes, thereby increasing the likelihood that a single bite will
cause infection. Yet, there are other concerns. |
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West Nile Virus Invades the United States
West Nile virus, transmitted to man primarily by mosquitoes,
was first isolated in 1937 in Uganda and later observed in the
Middle East, Asia, Oceania, and Europe. The virus was not
detected in the Western Hemisphere until 1999. Since then,
however, more than 3,000 infections have been reported in the
United States and more than 200 individuals have died.
Most infected people are never aware of the infection,
although some may develop flulike symptoms. But a small
percentage develop serious illness, including encephalitis and
spinal meningitis. There is as yet no preventive vaccine or
specific treatment available for West Nile virus. The U.S.
Centers for Disease Control and Prevention warns that West Nile
virus may also be acquired through organ transplants or a blood
transfusion from an infected donor. "There currently is no way
to screen blood for the West Nile virus," reported Reuters news
service in 2002.
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A Case Study in Disease
Changes in human society can also contribute to insect-borne disease.
To understand how, we need to take a closer look at the role of insects.
In many diseases an insect may be only one of several links in the chain
of disease transmission. An animal or a bird can serve as a host for a
disease by carrying insects on its body or by harboring microorganisms
in its bloodstream. If the hosts can survive this way, they may also
become a reservoir of the disease.
The deer tick (shown enlarged) spreads Lyme disease
to humans
Consider Lyme disease, identified in 1975 and named for Lyme,
Connecticut, U.S.A., where it was first observed. The bacterium that
causes Lyme disease may have come to North America a hundred years ago
with rats or livestock on ships from Europe. After a tiny Ixodes
tick ingests the blood of an infected animal, the bacteria remain in the
tick's gut for the rest of its life. When the tick later bites another
animal or a human, it can transmit the bacteria to the victim's
bloodstream.
In the northeastern United States, Lyme disease is endemic—it has
been present there for a long time. The main local reservoir of Lyme
disease bacteria is the white-footed mouse. The mice also serve as hosts
for the ticks, particularly ticks in their developing stages. Adult
ticks prefer to make their home on deer, where they feed and mate. Once
engorged with blood, the adult female tick drops to the ground to lay
her eggs, from which larvae soon emerge to begin the cycle anew.
A Shift in Circumstances
Pathogens have coexisted with animals and insects for many years
without causing disease in humans. But a change in circumstances can
turn an endemic disease into an epidemic—a disease affecting many people
in a community. What changed in the case of Lyme disease?
In the past, predator animals helped to limit contact between deer
ticks and man by controlling the deer population. When early European
settlers cleared the forests to farm, the deer population dwindled even
further and the deer's predators also moved on. But during the
mid-1800's, many farms were abandoned as agriculture moved westward, and
the forest began to reclaim the land. The deer came back, but their
natural predators did not. Thus, the deer population rebounded
explosively, and so did the tick population.
Some time later, the Lyme disease bacterium arrived and took up
residence for decades before emerging as a threat to humans. However,
when suburbs began to be built at the forest's edge, children and adults
in far greater numbers began to enter the ticks' domain. The ticks found
humans to attach themselves to, and the humans got Lyme disease. |
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Do Insects Spread HIV?
After more than a decade of investigation and research,
entomologists and medical scientists have found no evidence that
mosquitoes or any other insects transmit HIV—the AIDS virus.
In the case of mosquitoes, for instance, the insect's mouth
parts are not like a syringe with a single opening through which
blood could be reinjected. Rather, mosquitoes draw blood in
through one passage while delivering saliva through another.
Then, explains Thomas Damasso, an HIV specialist with the
District Health Management Team in Mongu, Zambia, the mosquito's
digestive system breaks down the blood, destroying the virus.
HIV is not found in insect feces. And unlike malaria parasites,
HIV does not get into the mosquito's salivary glands.
To acquire HIV, a person must be exposed to a large number of
infectious particles. If a mosquito's meal is interrupted and it
flies directly to another victim, any amount of blood that might
remain on its mouth parts would be too small to be significant.
According to experts, even swatting a mosquito filled with
HIV-positive blood over an open wound would not cause HIV
infection.
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"Almost all the new improved diseases owe their
comebacks to human meddling"
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Disease in an Unstable World
The foregoing scenario represents only one of the many paths taken by
disease and only one example of how the actions of man influence its
emergence. "Almost all the new improved diseases owe their comebacks to
human meddling," writes environmentalist Eugene Linden in his book
The Future in Plain Sight. A few other examples: The popularity and
speed of modern travel can spread pathogens and their carriers around
the globe. Damage to the habitats of creatures both large and small
threatens biodiversity. "Pollution flows into the air and water," notes
Linden, "weakening the immune systems of animals and humans alike." He
adds Dr. Epstein's summation: "In essence human tampering with ecology
has weakened the globe's immune system, fostering conditions favorable
for microbes."
Political instability leads to wars that damage ecosystems and
destroy the infrastructures that provide health care and food
distribution. Along with that, the Biobulletin of the American
Museum of Natural History points out: "Refugees, malnourished and weak,
are often forced into camps whose crowded and unsanitary conditions
expose people to a range of infections."
Economic instability drives human migration, both across and within
national borders, primarily into crowded urban areas. "Pathogens like
crowded places," explains the Biobulletin. As city populations
explode, "often essential public health measures, such as basic
education, nutrition, and vaccination programs cannot keep pace."
Overcrowding also places an extra burden on water, sewage, and
waste-disposal systems, making sanitation and personal hygiene difficult
while at the same time creating conditions that foster insects and other
disease carriers. Nevertheless, the situation is not hopeless, as the
following article will show.
Floods, unsanitary conditions, and human migration
contribute to the spread of insect-borne diseases
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